A thermistor is a thermally sensitive resistor whose primary function is to exhibit a change in electric resistance with a change in body temperature. Unlike a wire wound or metal film resistance temperature detector (RTD), a thermistor is a ceramic semiconductor. It has a metal sheathing (stainless steel or Inconel) and contains one or two thermocouple Sensing Wires (Chromel-Alumel-K Type or Chromel-Constantan-E type) running parallel to the metal sheathing and insulated from each other and sheathing by a ceramic insulating compound. Depending on the type of material used, a thermistor can have either a large positive temperature coefficient of resistance (PTC) or a large negative temperature coefficient of resistance (NTC).
Thermal sensors can detect temperature, infra-red source and its size, moving direction and speed, emissivity and wave length. As such, these can find applications in intruder alarms, fire alarms, laser detection, thermal recording, etc. (Sensors and Actuators, by MoonhoLee, Mina Yoo, A-96(2002) pp. 97-104). NTC sensors now a days are most commonly used in automotive applications (Sensors, Vol IV, by W. Gopel, J. Hesse, J. N. Zemel, Vol. 4, (1990)) and in precise temperature monitoring devices for temperature measurements, control and compensation. These sensors can provide precise temperature information at critical points. These type of sensors are reliable, stable, re-useable and maintenance free. A number of materials have been reported.
Thermistors are polycrystalline mixtures of sintered metallic oxides (NiO, Mn2O3, and Co2O3) or solid solutions (MgCr2O4 in Fe3O4) that behave essentially as semiconductors. As a result, they have negative temperature coefficients of resistance. They have proved successfully in a variety of shapes as small, inexpensive, sensitive, fast response temperature sensors, within the range—100° C. to 300° C. Thermistors for above 300° C. are made of oxides of rare earth elements, which are more refractory than nickel and manganese oxides and possesses higher activation energy. Thermistors for cryogenic use are mostly made from non-stoichiometric iron oxides which exhibit very low activation energy (Sensors, Vol IV, by W. Gopel, J. Hesse, J. N. Zemel Vol. 4, (1990)).
NTC thermistors consist of metal oxides, such as oxides of manganese, chromium, cobalt, copper, iron, nickel, titanium with different stoichiometric ratios, etc., and with different combinations (Measurements, Instrumentation and Sensors Hand Book, by Meyer Sapoff, p. 32.25). These exhibit a monotonic decrease in electric resistance with an increase in temperature. A number of papers have also been published with different combinations of La—Sr—Mn—O film La0.75Ca0.25MnO3, La2/3Sr1/3MnO3 pervoskite, etc., for magnetoresistive sensor (4,5,6,7). However, no studies so far have been reported with 95% Mn3O4 and 5% La2O3 mixture as NTC material.
Another reference may be made to Journal of Applied Physics by Sam Jin Kim and Chul Sung Kim, Vol. 91, No. 1 (January 2001) pp. 221-224.
Yet another reference may be made to Physics of Manganites by T. A Kaplan and S. D. Mahanti, 1998, p. 201.
One more reference may be made to Sensors and Actuators, by L I. Balcells, J. Cifre, A. Calleja, J. Fontcuberala, M. Varela and F. Benilez. 81 (2000) pp. 64-666.